1. Field of the Invention
The present invention relates to an image forming method which is used in recording or printing process such as electrophotographic processing, electrostatic printing and electrostatic recording.
2. Description of the Related Art
Hitherto, various types of electrophotographic processes have been known such as those disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, corresponding to U.S. Pat. No. 3,666,363 and Japanese Patent Publication No. 43-24748, corresponding to U.S. Pat. No. 4,071,361. In general, these known electrophotographic processes employ a photoconductive material on which an electrical latent image is formed by various means. The latent image is then developed into a visible image by means of a toner and the developed image is transferred as required to a transfer member such as a sheet of paper, followed by fixing which is conducted by application of heat, pressure, heat and pressure or solvent vapor, whereby a copy image is obtained.
Developing methods in which images are developed under influence of a bias voltage are disclosed in U.S. Pat. Nos. 3,866,574, 3,890,929 and 3,893,418.
A method also has been proposed which uses a high-resistance mono-component toner, wherein a specific gap is preserved between a latent image carrier and a toner carrier and an asymmetric alternating pulse bias voltage is applied between the latent image carrier and the toner carrier so as to control conveyance of the toner. FIG. 9 schematically shows the waveform of the alternating pulse bias voltage used in this control method. More specifically, in this method, the gap between the latent image carrier and the toner carrier is approximately 50 to 500 .mu.m, preferably 50 to 180 .mu.m, and the frequency of the pulse bias voltage is approximately from 1.5 to 10 KHz, preferably 4 to 8 KHz. The developing time T.sub.A is approximately from 10 to 200 .mu.sec, preferably from 30 to 200 .mu.sec, while peeling or reverse-development time T.sub.D in which the toner is peeled off the latent image carrier is set to from about 100 to 500 .mu.sec, preferably from 100 to 180 .mu.sec. The developing voltage is determined to be lower than about -150 V, preferably between -150 V and -200 V, while the reverse-development or peeling voltage, which is of inverse polarity to the developing pulse and which acts to peel the toner off the latent image carrier, is determined to be higher than about 400 V, preferably between 400 V and 450 V.
This method effectively improves gradation and reproducibility while preventing deposition of the toner being conveyed to non-image area of the image carrier. FIG. 10 schematically illustrates the manner in which particles of the toner are conveyed.
Thus, in the above-described developing method, the absolute value of the alternating bias voltage is set to a low level and the developing voltage also is set to a low level, in order to prevent deposition of the toner particles to a non-image area. Unfortunately, however, this developing method often fails to provide high density of the developed images. There are some known developing methods which utilize high-resistance mono-component developing agents having volumetric resistance not lower than 10.sup.10 .OMEGA.cm. Examples of such methods are a so-called impression developing method as disclosed in U.S. Pat. No. 3,405,682 and a so-called jumping developing method as disclosed in Japanese Patent Laid-Open Nos. 55-18656 through 55-18659. In the jumping developing method, alternating bias voltage applied between the toner carrier and the latent image carrier causes the toner to reciprocate therebetween within the developing region where the distance between the toner carrier and the latent image carrier is smallest. The toner finally attaches selectively to the latent image carrier surface in accordance with the pattern of the latent image, thus developing the latent image into a visible image. As will be seen from FIG. 11, the alternating bias voltage has a duty ratio of 50%, i.e., the duration of the developing voltage component which acts to deposit the toner onto the latent image carrier surface and the duration of the peeling or reverse-development voltage component acting to peel the toner are equal to each other.
In a specific form of this jumping developing method, the duty ratio of the alternating bias voltage applied between the toner carrier and the latent image carrier is controlled in accordance with the amount of the toner remaining on the toner carrier, thereby allowing the density of the developed image to be altered as required, as disclosed in Japanese Patent Laid-Open No. 60-73647.
Copy images produced by the developing methods which utilize high-resistance mono-component toner generally exhibit small degrees of gradation due to the fact that the high-potential region of the latent image is developed at a high density by virtue of the high developing voltage component while low-potential region of the latent image is not developed satisfactorily because the toner is excessively peeled off the latent image carrier due to application of an unduly high reverse-development voltage component of the alternating bias pulse voltage. Another drawback of this method is that the tolerance for setting the developing voltage component, which has a direct current (D.C.) component and an alternating current (A.C.) component, is impractically small. Namely, an attempt to raise the density level by lowering the level of the D.C. component or elevating the level of the A.C. component tends to cause fogging in white blank areas. Increasing the frequency of the A.C. component is an effective measure for suppressing generation of fog but this method seriously deteriorates reproducibility due to excessive thinning of character and line images.
In order to overcome the above-described problems, a method has been proposed in which the level of the developing electric field during application of the developing voltage component is enhanced and the duration of this component is shortened, thereby simultaneously attaining high image density, high gradation and good image quality without fog.
It has been noted, however, that this proposed method is still unsatisfactory in that it allows a deterioration of the image quality such as a reduction in the image density and increase in the fog, as well as degradation in resolution and line reproducibility, when this developing method is executed repeatedly for a long period of time. It has been proved that the deterioration of the image quality is attributable to a change in the particle size distribution of the toner caused by selective consumption of toner particles during long use.
One of the advantageous features of the developing devices which perform development by the previously described developing method is that the size of such developing devices can be made appreciably small, which allows margin spaces to be generated around the photosensitive member as the latent image carrier, particularly in high-speed copying machines. This enables a plurality of such small developing devices having color toners other than black to be disposed around the photosensitive member so as to make it possible to change the recording color by a simple change-over operation. Furthermore, by employing this developing material it becomes easier to simultaneously conduct formation of latent images by an analog light, formation of latent images of page numbers and characters by laser light and to simultaneously develop these latent images.
The toner used in the developing method of the type described is required to have higher stability in the charged state against environmental conditions than other types of toners, in order to attain superior quality, durability and stability of the copy images.
Furthermore, the current trend for higher speed of operation of copying machines have given rise to a demand for toners which satisfy various requirements such as high resolution, high developing speed and superior durability. Studies are being made to develop toners which satisfy such requirements.
Among various types of toners, a toner known as magnetic toner contains a magnetic material which occupies a large part, e.g., 20 to 70 wt %, of the whole toner. Thus, the performance of magnetic toner significantly depends on the nature of the magnetic material.
A magnetic toner containing 16 to 25 wt % of FeO as magnetic powder, which is disclosed in Japanese Patent Laid-Open No. 58-189646 corresponding to U.S. Pat. No. 4,946,755, offers high efficiency development of electrostatic latent images, as well as high efficiency of image transfer, and ensures a high degree of stability of the toner image. However, it is not easy to attain high degrees of resolution, developing speed and durability with this type of magnetic toner, particularly when this type of magnetic toner is used in a high-speed copying machine which produces 50 or more copies per minute. Namely, when this type of magnetic toner is used in such a high-speed copying machine, a difficulty is encountered in controlling the amount of charges on the magnetic toner, particularly in an environment of low temperature and low humidity. Consequently, reduction in the image density and fogging of the background are often experienced due to excessive charging of the magnetic toner. One measure for preventing excessive charging of the magnetic toner is to increase the content of the magnetic material in the magnetic toner. This solution, however, impairs fixing performance and, hence, is not preferred from the view point of application to high-speed copying machines.
Various methods and devices have been developed also for fixing toner images to sheets such as copy papers. They include the heat-press type fixing method and a device employing heat rollers. The heat roller has a surface which is repellent to toner. A sheet carrying a toner image is conveyed such that its image carrying surface is pressed by the toner-repellent surface of the heat roller, whereby the toner image is fixed. According to this method, since the heat roller surface makes a pressure contact with the toner image, the toner can be fused and fixed to the sheet at high efficiency, thus enabling a quick fixing of the image. This type of fixing method, therefore, can suitably be used in high-speed copying machines.
In order to further improve fixing performance in this type of fixing method, Japanese Patent Laid-Open No. 55-134861, corresponding to U.S. Pat. No. 4,504,563, proposes use of a toner containing a binding resin having an acidic component. This type of toner, however, is too sensitive to changes in environmental conditions such that it tends to be charged either insufficiently and excessively, when the humidity of the ambient air is high and low, respectively.
The presence in a toner of an acid anhydride groups serves to improve chargeability. With this knowledge, Japanese Patent Laid-Open Nos. 59-139053 and 62-280758 propose toners which contain a binding resin formed from a polymer having many acid anhydride groups. The polymer is mixed with and diluted by a different type of resin. This type of toner essentially requires that the resin having acid anhydride groups is uniformly dispersed in the binding resin, for otherwise undesirable effects, such as fogging, tend to occur during development due to non-uniform mutual charging of the toner particles. In addition, the resin binder of the type described above exhibits an unduly strong negative charging characteristic and, hence, cannot be used in toners having positive charging characteristic.
Further, Japanese Patent Laid-Open Nos. 61-123856 and 61-123857 propose a method in which acid anhydride group units are dispersed, through copolymerization, in the polymer chains of the binding resin. Toners produced by this method exhibit superior fixing characteristics, as well as anti-offset and developing performance, but are liable to be charged excessively, particularly when used in high-speed machines in air of low humidity, thereby causing fogging and reduction in image density. One of the causes for such excessive charging is that, although the binding resin has abundant acid anhydride group units, these units are not dispersed uniformly.